Method for the development of latent electrostatic images

ABSTRACT

A method for developing electrostatic latent images comprises applying a developer, which comprises a liquid or paste vehicle containing toner particles, to an electrostatically charged surface from a roller or other applicator. The liquid or paste developer is applied in a very thin film to the roller so that substantially the only toner particles contained in the liquid or paste film are in the electrical double layer formed at the roller - liquid (paste) interface. The applicator, or at least the outer surface thereof, is made of a material with a dielectric constant between about 100 and 10,000 preferably a titanate of a bivalent metal, e.g., barium titanate or calcium titanate (perovskite). The developer is preferably a thixotropic dispersion of polarizeable toner particles in a non-polar liquid.

United States Patent [191 Kosche Mar. 19, 1974 METHOD FOR THE DEVELOPMENT OF LATENT ELECTROSTATIC IMAGES [76] Inventor: Horst Kosche, Feldstrasse 21,

Dueren-Lendersdorf, Germany [22] Filed: Apr. 6, 1972 [21] Appl. No.: 241,824

[30] Foreign Application Priority Data Apr. 9, 1971 Austria 3075/71 [52] US. Cl. ..117/37 LE, 96/1 R, 117/111 F, 96/1 LY, 355/10 [51] Int. Cl G03g 13/10 [58] Field of Search..... 117/37 LE, 111 F; 118/637, 118/DIG. 23; 96/1 R, l LY; 355/10 [56] References Cited UNITED STATES PATENTS 3,084,043 4/1963 Gundlach 117/37 LE 3,292,572 12/1966 -Brenneisen et al..... 118/637 3.425.829 2/1969 Cassiers et al. 117/37 LE 3,432,329 3/1969 Heuvelet al 117/37 LE Primary Examiner-Murray Katz Assistant Examiner-M. Sofocleous Attorney, Agent, or Firm-Meyer, Tilberry & Body [5 7] ABSTRACT A method for developing electrostatic latent images comprises applying a developer, which comprises a liquid or paste vehicle containing toner particles, to an electrostatically charged surface from a roller or other applicator. The liquid or paste developer is applied in a very thin film to the roller so that substantially the only toner particles contained in the liquid or paste film are in the electrical double layer formed at the roller liquid (paste) interface. The applicator, or at least the outer surface thereof, is made of a material with a dielectric constant between about 100 and 10,000 preferably a titanate of a bivalent metal, e.g., barium titanate or calcium titanate (perovskite). The developer is preferably a thixotropic dispersion of polarizeable toner particles in a non-polar liquid.

13 Claims, No Drawings METHOD FOR THE DEVELOPMENT OF LATENT ELECTROSTATIC IMAGES The present invention is concerned with a method for the development of electrostatic charge latent images by the application to the surface containing the electrostatic charges of particles suspended in a liquid or paste medium. More particularly, the invention concerns a method usually referred to as liquid development, for the development of electrostatic latent images by means of roller application of a thin film of a liquid or paste developer containing polarizeable toner particles.

Liquid development of electrostatic images thus refers to a process in which a'suspension of charged toner particles in a liquid (or paste) vehicle is caused to migrate through the vehicle onto a suitably charged surface. The charged surface contains electrostatic charges, usually formed in a pattern so that development of the charged (or alternatively, the uncharged) portion of the charged surface by attraction of the toner particles thereto results in an image being formed. The pattern of electrostatic charges, usually obtained by exposure of a charged photoconductive surface to light reflected from an image, or its negative, thus comprises an electrostatic latent image. ldeally, the surface upon which the image is to be developed will accept the toner particles and repulse the vehicle.

The suspended particles, when dispersed in the liquid or paste, acquire an electric charge because of the generation of an interfacial contact potential difference between the surface of the toner particle and the liquid. An electric field may be imposed to increase the rate of migration of the charged toner particles to the surface bearing the electrostatic-charge latent image.

As is generally known, any solid in contact with a liquid acquires a uniformly distributed charge over its surface by generation of an interfacial potential difference. The charge is transferred across the interface by electron movement and may be positive or negative on the solid, depending on the respective natures of the solid and the liquid. According to the well known electrical double layer theory, an opposite charge is attracted to the region of the liquid surrounding or facing the charged solid-liquid interface, so that a double layer of interface electrical charges is formed. It is further believed that the outer (liquid side) layer of charges is diffused somewhat by thermal agitation and movement of the liquid particles, so that a somewhat diffuse cloud of charges is present at the interface.

The velocity of movement of the charged toner particles through the vehicle is generally accomplished under the influence of the electromotive force of an imposed electric field. However, the strength of the field which can be applied, and consequently the velocity of migration of the charged particles, is limited because if stronger fields are applied to attempt to increase the velocity of migration, further polarization of the charged particles is promoted. This results in chain forof this and of the generally low migration speed of the particles, the rate of deposition of toner particles on the charged image surface is rather low and proceeds only intermittently at any one given spot on the image surface.

The present invention provides a method for developing latent electrostatic images comprising collecting toner particles in a boundary layer adjacent a solid applicator, discharging or removing the charge cloud which would otherwise surround the boundary layer, and bringing the applicator with its boundary surface of high charge density into contact with the surface carrying the electrostatic charge latent image, or so close to it, that the densely packed boundary layers of toner particles are deposited as a strata of toner particles on the corresponding portion of the electrostatically charged surface.

In accordance with another aspect of the invention, the intimate contact between the densely packed boundary layer charge and the surface induces on the surface carrying the electrostatic latent image new charges, which increase the acceptability of additional toner particles to form the image.

The method of the invention may be carried out in a particularly advantageous manner by treating a dispersion of toner particles in a liquid vehicle, prior to bringing the same into contact with the surface containing the latent electrostatic image to be developed, to remove or discharge the statically charged particles not contained in the controlled boundary layer of charges at the solid-liquid interface. This may be accomplished, as described in detail below, by mechanical arrangements or by charged flow, friction, or triboelectric potentials. By discharging or removing the cloud of static charges and charged particles, a high density boundary layer of charged toner particles formed between an applicator and the liquid vehicle is brought into contact with, or very close to, the surface carrying the latent electrostatic image. The boundary layer of charged unipolar toner particles are thus transferred in extremely thin layers in a thin film of liquid vehicle to the electrostatically charged surface. The toner particles are thus deposited in a densely packed layer or strata onto the suitably charged portions of the electrostatic latent image.

Further, prior to the deposition of the polarized toner particles, new charges suitable for the acceptance of additional toner particles are thereby induced on the electrostatically charged surface to be developed.

A simple and convenient means for carrying out the method of the invention comprises utilizing a solid body, for example in the form of a cylinder, which is brought into contact with a dispersion of toner particles contained in a liquid medium, the cylinder being free from surface charges by grounding or other suitable means, thereby forming an electrically charged double interfacial layer on the wetted portions of the cylinder. The thickness of the wetted layer is determined by the charge of the boundary surface (of the solid cylinder) and by the geometric condition of the solid boundary surface, i.e., its roughness, perosity, etc. The electric charges at the interface between the solid cylinder and the liquid vehicle decrease with increasing distance into the liquid away from the solid-liquid interface.

The liquid utilized as a liquid vehicle should be nonpolar with a low dielectric constant, but being capable of being reversibly polarized. It is also desirable that the liquid vehicle is not miscible with polar substances, the presence of which would be deleterious to the invention. For example, substances with high dipole moments, e.g., water, NH OH, etc. should be avoided. Accordingly, it reduces the chances of contamination if such polar substances are not miscible in the non-polar, high dielectric constant liquid vehicle.

Since the operation of the electrostatic toner system of the invention depends upon the formation of a charged double layer between the applicator and the liquid developer, without extraneous charged particles remaining in the liquid outside the boundary layer, it is important that the charges formed are relatively immobile on the applicator, until brought into contact with the latent electrostatic image.

In operation in accordance with the invention, a boundary layer of charged toner particles is formed on the boundary between the solid (preferably cylindrical) applicator and the liquid vehicle clinging thereto, extraneous (non-boundary layer) charged particles are removed, and the boundary layer is brought into contact or very close proximity to the surface bearing the electrostatic charges forming the latent image. The contact or close proximity can be achieved by mechanical movement or by charged, friction or triboelectric flow techniques. The polarized layers of toner in the liquid vehicle are thereby deposited by strata upon the surface to be developed.

Because substantially all toner particles and other static charges which are not contained in the polarized boundary layer are removed by means such as a wiper or scraper blade, etc., a developer system which is particularly effective is attained. It is capable of reproducing images from extremely weak electrostatic charges which can no longer be developed with non-polarized toner systems, and can operate by free surface charge attraction only, i.e., without an imposed electric field.

Any suitable means may be employed to remove sub stantially all components ofthe toner and other charges not polarized in the boundary layers. For example, scrapers, wipers, doctor blades," air currents, gravitation, centrifugal force, etc. may be employed to remove non-boundary layer liquid from the applicator.

By thus removing all excess liquid or paste vehicle, substantially all extraneous toner particles, charges and adsorbed polar substances are removed so that they do not interfere with the development process.

By induction of the charged polar layers which cling to the solid body or applicator and by the induction of charges by the applicator itself, the layer to be developed has induced upon it additional charges which reinforce the strength of the latent electrostatic image.

The method of the invention thus provides particularly intensive development of electrostatic charges forming a latent image and is particularly suitable for the formation of half tones and gradations of tone intensities by offering to the charged surface to be developed extremely thin coatings of toner particles.

Multiple images may be obtained by the method of the invention by simply repeating the process.

In carrying out the method of the invention, it is suitable to use a solid applicator which can accomplish, along various portions of its surface, the different functions required, and which can perform these functions by continuous motion in multiple passes. A cylindrical form is suited to this type of operation. By employing a very thin film of the developer liquid or paste on the applicator the attraction of the toner particles to the surface to be developed and the repulsion of the liquid vehicle is facilitated, whereby particularly intensive development is achieved.

The invention has particular utility in those arrangements wherein developer systems of such high viscosity are employed that the field intensity of the latent electrostatic image no longer suffices, when prior art methods are used, for attracting the toner particles out of the highly viscuous carrier liquid, depositing them upon the surface to be developed, and repulsing the paste or liquid carrier.

The developers contemplated here are usually of such high viscosity that they are no longer truly liquids but appear to have a paste-like consistency. Pasty developers are highly advantageous and allow the application of electrophotography in areas where it was previously not applicable. For example, a paste rather than a liquid permits the use of cartridges in portable copying devices and cameras, so that immediate reproduction of an image may be attained.

Morover, it is advantageous in certain applications to produce a pasty consistency of the developer totally or partially by the use of thixotropic polarizable substances. Thixotropic materials which carry different charges on different, adjacent surfaces are particularly valuable for this use. If the thixotropic thickening materials possess in addition a sufficient mobility in the developer so that upon the application of a polarizing field or upon the formation of electric double layer on a solid body, they induce under polarized orientation an additional auxiliary field in more remote layers of developer, and make these polarized, more remote layers more capable of becoming displaceable in themselves as developing stratas of toner.

The selection ofa particular polarizing solid body for the applicator depends upon the polarity of the toner selected. In the direct development or toning of surfaces containing the latent electrostatic image (i.e., the photo conductor insulating layers) with a negative charge, toner particles with a positive charge are employed, in which process the liquid portion of the boundary surface is negatively charged.

Particularly for the use of paste developers, applicator materials with a dielectric constant between and 10,000 are particularly valuable. They may naturally also be used with more liquid developers. For example, the perovskites or similar substances such as (IV) titanates of bivalent metals are preferred. Among these are barium titanate and calcium titanate (perovskites). These materials usually have a dielectric constant between about 500 and 1,000, although it may range anywhere from l00 up to 10,000. The entire body of the applicator need not necessarily be composed of such materials, but it may suffice to have the surface of the applicator formed by the high dielectric material.

(The reference to dielectric constant has of course its usual meaning as being the ratio of the permittivity of the dielectric to the permittivity of vacuum).

Also, the polarizing forces of the solid body can be intensified by polarization in a condenser or by the application of auxiliary electric fields.

Examples in accordance with the invention are set forth below.

EXAMPLE I A direct developer for negative charges is produced whose toner medium is formed by the asphalt phase of bitumen and adsorbed soot particles, and in which the dispersant consists of a high boiling kerosene fraction or paraffin oil which is compounded with additional paraffins to a flash point (FP) of 59 to 63. This is either a solid or a material having a tendency of crystallizing at room temperature. The material is melted by applying heat as required to form a highly viscous, not yet crystallizing paste upon cooling. Into this paste the toner particles are worked on a roller mill and additional amounts of kerosene and a synthetic resin of the polyterpene group are added, so that a specimen which no longer drips off a glass rod dipped into the compound is obtained.

An applicator roller made of perovskite (calcium titanate) has its surface slightly roughened by sand blasting, and is dipped into a trough containing the toner paste so that the surface of the roller is completely wetted.

The toner paste is now wiped with a blade scraper in such a manner that only an extremely thin film remains upon the surface of the roller. By continuously rotating the roller, this film is brought into contact with the surface of a band of negatively charged paper which is drawn over the roller.

A visible record is thus produced upon the paper in accordance with the electrostatic charge carried thereon. This developer arrangement is particularly useful for providing a clean, easily handled cartridge which can be used in portable copying devices or cameras for immediate re-production of the image.

EXAMPLE II A positively charged toner medium is produced in accordance with Example l, and dispersed in a highly refined kerosene. A thixotropic substance such as silicic acid, kaolin or bentone is added in sufficient quantities that a paste which will no longer drip from a glass rod is formed. The image is developed according to the process described in Example I, and a visible image of a negative electrostatic charge is produced.

The invention has been described with respect to particular embodiments thereof. However, upon reading and understanding the foregoing, certain modifications and variations will become apparent to those skilled in the art. It is intended to include all such modifications and variations within the scope of the appended claims. Further, the term liquid as used in the claims is intended to include both low and high viscosity liquids and pastes, as discussed above.

What is claimed is:

1. A method for the development of latent electrostatic charge images comprising applying a non-polar liquid containing toner particles dispersed therein to a solid applicator body, at least the outer surface of which has a dielectric constant between about and 10,000, removing excess liquid so that only a thin liquid film remains on the body, said liquid film containing a boundary layer of charged toner particles adhering to the solid body and being substantially free of nonboundary layer charged toner particles, bringing said thin film of liquid containing the boundary layer into close proximity with a surface bearing an electrostatic charge, and transferring the charged toner particles from the solid applicator body to the charged surface to form an image thereon.

2. The method of claim 1 wherein said liquid is not miscible with polar substances.

3. The method of claim 1 where said liquid is capable of being reversibly polarized.

4. The method of claim 3 wherein the applicator body has a dielectric constant of between about 500 and 1,000.

5. The method of claim 1 wherein at least the surface portion of said applicator body which has said liquid film formed thereon is made of a titanate of a bivalent metal.

6. The method of claim 5 wherein said surface portion is made from a material selected from the class consisting of barium titanate, calcium titanate and mixtures thereof.

7. The method of claim 1 wherein a thixotropic thickening material is added to the liquid vehicle.

8. The method of claim 7 wherein the thixotropic thickening material is a polarizible thixotropic material.

9. The method of claim 8 wherein the thixotropic thickening material is selected from the class consisting of silicic acid, kaolin, bentone and mixtures thereof.

10. The method of claim 7 wherein the thixotropic thickening material is added in sufficient quantity to form a paste.

11. The method of claim 1 including repeating each step therein.

12. A method for the development of a latent electrostatic image comprising applying a pasty, thixotropic dispersion of polarizable toner particles in a non-polar vehicle upon a roller, at least the outer surface of which is made of material with a dielectric constant between about 100 and 10,000 wiping the roller to leave a thin film of said dispersion thereon, which dispersion is substantially free of charged toner particles not contained in a dense boundary layer of charged toner particles at the roller-dispersion interface, bringing the charged toner particles carried on said roller into close proximity with a surface containing an electrostatic latent image by moving the electrostatic image surface relative to said roller, thereby transferring toner particles from the roller to the electrostatic image surface.

13. The method of claim 12 including repeating each step therein. 

2. The method of claim 1 wherein said liquid is not miscible with polar substances.
 3. The method of claim 1 where said liquid is capable of being reversibly polarized.
 4. The method of claim 3 wherein the applicator body has a dielectric constant of between about 500 and 1,000.
 5. The method of claim 1 wherein at least the surface portion of said applicator body which has said liquid film formed thereon is made of a titanate of a bivalent metal.
 6. The method of claim 5 wherein said surface portion is made from a material selected from the class consisting of barium titanate, calcium titanate and mixtures thereof.
 7. The method of claim 1 wherein a thixotropic thickening material is added to the liquid vehicle.
 8. The method of claim 7 wherein the thixotropic thickening material is a polarizible thixotropic material.
 9. The method of claim 8 wherein the thixotropic thickening material is selected from the class consisting of silicic acid, kaolin, bentone and mixtures thereof.
 10. The method of claim 7 wherein the thixotropic thickening material is added in sufficient quantity to form a paste.
 11. The method of claim 1 including repeating each step therein.
 12. A method for the development of a latent electrostatic image comprising applying a pasty, thixotropic dispersion of polarizable toner particles in a non-polar vehicle upon a roller, at least the ouTer surface of which is made of material with a dielectric constant between about 100 and 10,000 wiping the roller to leave a thin film of said dispersion thereon, which dispersion is substantially free of charged toner particles not contained in a dense boundary layer of charged toner particles at the roller-dispersion interface, bringing the charged toner particles carried on said roller into close proximity with a surface containing an electrostatic latent image by moving the electrostatic image surface relative to said roller, thereby transferring toner particles from the roller to the electrostatic image surface.
 13. The method of claim 12 including repeating each step therein. 